Before discussing the application differences, it may be worth comparing the different chemistries involved:
Offset inks that ‘dry’ through a combination of absorption into the paper matrix and film formation, due to the oxidation of the so called ‘drying oils’ in the ink formulation, have been around for many decades and their use, behaviour and numerous ‘quirks’ should be understood by every sheetfed offset press operator. These ‘conventional’ ink formulations comprise principally of the pigment, drying oils, hydrocarbons, resins and polymers plus various specialised additives such as dryers, waxes and surface-active agents.
UV inks that dry by the UV initiated curing of a monomer are completely different. Perhaps the only component that they share in common with conventional inks is the pigment. Otherwise, they comprise of a monomer, a photo-initiator and perhaps some small volume of an oxygen containing solvent.
How does this difference manifest in terms of on-press usage?
Perhaps the biggest ‘lithographic’ difference is in emulsion formation. The wetting characteristics of UV ink films are very different to those of conventional inks. UV curing ink surfaces are generally easier to wet than those of conventional inks, which results in an increased propensity to form undesirable ink in water emulsions on the press. Indeed, this is the reason why it is more difficult to run isopropanol-free with UV inks, particularly at higher printing speeds.
Another important difference is in the reaction between the ink components and the various substrates that they come into contact with. Conventional inks contain oils and hydrocarbon solvents that have no damaging effect (swelling and softening followed by shrinking, hardening and potentially cracking) on butyl or nitrile rubbers but act as solvents for EPDM rubbers. UV ink monomers act like solvents for nitrile and butyl rubbers but have little or no effect on EPDM rubbers. Furthermore, the UV monomers also act as solvents for many positive working offset plate coatings unless the plates are ‘baked’ after exposure and development.
These two factors mean that the two ink technologies must be handled very differently in the pressroom. This is easy to achieve when printing with either conventional or UV inks. That is, with dedicated presses for each, but presents many issues when alternating between the two technologies on the same press.
Regarding undesirable emulsions and the associated toning that often results, accurate control of ink and water feeds as well as dampening temperature is vital. IPA should be at a minimum of 8%. Dedicated UV ink fountain solutions are available and definitely help with these issues however they may cause issues with conventional inks such as retarded drying or positive working plate premature wear.
With regard to offset plate compatibility, it is important to realize that the ink itself can act as a solvent for the coating. The tried and tested method to resolve this issue is baking which will render the image areas impervious to chemical attack of any kind. Most plate manufacturers now offer so called UV ink resistant positive working plates and indeed these are much more resistant to the effects of UV inks than conventional offset plates but nevertheless are not 100% impervious. Achievable run lengths will increase but not to the extent achieved by baking.
It should be clear by now that the simplest option when running UV is to dedicate a press to this function, running plates, founts, washes, blankets and rollers that are UV specific. However, it is recognised that this option is often not commercially viable and so therefore advice for those printers wishing to run both types of inks on one press is required.
Ultimately, the word here is ‘compromise’. It must be recognised that one can never enjoy the benefits of running separate dedicated presses for each type of ink technology.
So where are these compromises to be made?
Most manufacturers of blankets and rollers recognise this issue and therefore produce products made with compounds that are optimised for maximum lifespan when using a mix of UV and conventional, the question here is ‘what mix’? 80:20? 50:50? 20:80?
Clearly a product optimised for 50:50 split usage may give issues if the split is 80:20 so caution is advised. Discussing exact requirements with the supplier is always a good idea! This advice would also apply to plate choice. Negative working plates should in general have no issues here but with positive working, plates preparation or selecting them for the specific job is important.
Running UV? Then either bake or use a UV resistant type.
Particular attention should be paid to the choice of roller wash as an irreparably damaged roller train can be expensive to replace. I have witnessed the effect of using a UV roller wash on a press with butyl rubber rollers, the whole roller train on each unit were in effect glued together and a complete new set of rollers was required as well as two days of work removing them!
The situation is somewhat easier when selecting an appropriate fountain solution concentrate. The adage that ‘any fount will run on any press’ is true so long as complete freedom from issues is not the most important consideration. Certainly, alcohol and temperature control are more critical with UV but with care most good quality founts designed for conventional should run with UV albeit at slightly lower pan temperatures and somewhat higher alcohol concentrations.